The Collaborative International Dictionary of English v.0.48:

Gallinule \Gal"li*nule\, n. [L. gallinula chicken, dim. of
   gallina hen: cf. F. gallinule.] (Zool.)
   One of several wading birds, having long, webless toes, and a
   frontal shield, belonging to the family Rallidae. They are
   remarkable for running rapidly over marshes and on floating
   plants. The purple gallinule of America is Ionornis
   Martinica, that of the Old World is Porphyrio porphyrio.
   The common European gallinule (Gallinula chloropus) is also
   called moor hen, water hen, water rail, moor coot,
   night bird, and erroneously dabchick. Closely related to
   it is the Florida gallinule (Gallinula galeata).
   [1913 Webster]

   Note: The purple gallinule of Southern Europe and Asia was
         formerly believed to be able to detect and report
         adultery, and for that reason, chiefly, it was commonly
         domesticated by the ancients.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water hen \Wa"ter hen`\
   1. (Zool.) Any gallinule.
      [1913 Webster]

   2. (Zool.) The common American coot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water hen
    n 1: a coot found in North America [syn: American coot, marsh
         hen, mud hen, water hen, Fulica americana]
    2: any of various small aquatic birds of the genus Gallinula
       distinguished from rails by a frontal shield and a
       resemblance to domestic hens [syn: gallinule, marsh hen,
       water hen, swamphen]